Operating an mri apparatus

ABSTRACT

A method of operating a magnetic resonance imaging (MRI) apparatus includes exciting a body coil of the MRI apparatus to emit a radio-frequency signal, determining a center frequency of a resonance curve of the body coil, and calculating a magnet target frequency based on the determined center frequency. A magnet is ramped to the magnet target frequency.

This application is a continuation patent application of U.S. patentapplication Ser. No. 17/386,533, filed on Jul. 27, 2021, which is adivisional patent application of U.S. patent application Ser. No.16/546,296, filed on Aug. 20, 2019, which claims the benefit of EuropeanPatent Application No. EP 18189940.2, filed on Aug. 21, 2018, EuropeanPatent Application No. EP 18189973.3, filed on Aug. 21, 2018, andEuropean Patent Application No. EP 18189964.2, filed on Aug. 21, 2018,which are hereby incorporated by reference in their entirety.

BACKGROUND

The present embodiments relate to operating a magnetic resonance imaging(MRI) apparatus.

Developments in the field of MRI systems have led to advances inlow-field systems, which may be preferred on account of a smallerfootprint. Low-field systems may be open-bore, may allow interventionalprocedures, and are less expensive. The term “low-field system”generally refers to a system that has a magnetic field strength of atmost 1.0 Tesla. An MRI system with a magnetic field strength in excessof 1.0 Tesla is generally referred to as a “high-field system.” Themagnetic field strength of the low-field systems currently indevelopment may be even lower than 1.0 Tesla, and may even be lower than0.5 Tesla.

When an MRI system is first installed on site, a ramping procedure iscarried out to set up the main magnetic field (also referred to as thestatic background field) in the main coil windings. After the initialinstallation ramping procedure, shim coils are used to perform anyadjustments necessary to take into account the local environment.Usually, the target frequency (e.g., the center frequency of the mainmagnet) is determined with the aid of a probe placed at a suitableposition in the apparatus. To allow for the inevitable decay of themagnetic field arising from component aging, typically in the order ofseveral hundred ppm per year, this target frequency generally exceedsthe center frequency of the body coil by an amount that is sufficient toprovide that the center frequency of the main magnetic field remainsabove the body coil center frequency for as long as possible.

In any superconducting MRI system, decay of the magnetic field isunavoidable due to the residual electrical resistance of the magnet.Field decay provides that the center frequency of the main magneticfield gradually drifts away from the initial setting. Ultimately, itbecomes necessary to re-ramp the system. In a high-field system, thebandwidth of the body coil and radio-frequency system is so large (e.g.,body coil bandwidth ±100 kHz or more) that it may take several yearsbefore field decay is out of specification. Generally, another serviceprocedure such as a cold head exchange is to be scheduled sooner. Tocarry out such a service procedure, the magnet is to be ramped down andup, so an opportunity is given to re-calibrate the system.

Before ramping the magnet, a target frequency for the main magneticfield is identified. It is usual to set the highest possible targetfrequency and to use the shim coils for any fine-tuning of the mainmagnetic field. By setting the highest possible target frequency, thedecay window (e.g., the time taken for the frequency to drift to thelower end of the allowed band) is made as long as possible. Thisapproach is suitable for systems with a high bandwidth, as explainedabove. However, the bandwidth of a low-field system is significantlynarrower than that of a high-field system, being only on the order of 10kHz-25 kHz, so that prior art methods of setting the magnet targetfrequency are limited to a much shorter “decay window.” Because thedecay window is narrower, the magnet frequency of a low-field MRI systemdecays towards an out-of-spec level in a shorter space of time. Thisprovides that low-field MRI systems are generally to be ramped moreoften.

The first superconducting low-field MRI systems were generallyconfigured as vertical field systems, characterized by a relativelyinefficient body coil. These early low-field vertical systems are knownto be less reliable than a comparable horizontal field system (e.g.,with a birdcage body coil), so that service checks are to be scheduledrelatively frequently. During these service procedures, the frequency ofthe main magnetic field is checked and the system is re-ramped ifnecessary. In a low-field system, it may be necessary to carry out anintermittent ramping procedure (e.g., in the event of an infrastructureproblem such as a loss of power or problems with a cooling issuesarrangement, etc.).

Ideally, the center frequency of the body coil would be the same as thefrequency of the main magnet field. However, with a low-field magnet,reflection effects reduce the accuracy of the system, and the centerfrequency is lower.

The narrow bandwidth of the radio-frequency system, especially the bodycoil and the receiver coils, provides that reflection coefficientssignificantly reduce the available power from the radio frequency poweramplifier (RFPA). When the reflection coefficients from the body coilare not equal, power is partly reflected back into the RFPA, which leadsto a derating of the RFPA. This reduces the available power for theradio frequency magnetic field generated by the transmit coil or bodycoil. This magnetic field is generally referred to as the B1 field.Furthermore, the center frequency of the magnet and the center frequencyof the body coil are not necessarily identical. For these reasons, themagnet center frequency is to be greater than the body coil centerfrequency to allow for inevitable main magnet field decay over time.However, it is difficult to identify a magnet target frequency thatworks satisfactorily within the constraints of narrow body coilbandwidth and high reflection coefficients.

SUMMARY AND DESCRIPTION

The scope of the present invention is defined solely by the appendedclaims and is not affected to any degree by the statements within thissummary.

The present embodiments may obviate one or more of the drawbacks orlimitations in the related art. For example, a way of ramping a magneticresonance imaging (MRI) apparatus that overcomes problems outlined aboveis provided.

According to an embodiment, a method of operating an MRI apparatusincludes exciting a body coil of the MRI apparatus to emit aradio-frequency signal, determining a center frequency of a reflectedradio-frequency signal, calculating a magnet target frequency based onthe determined center frequency, and ramping a magnet to the magnettarget frequency.

The magnet is ramped at a scheduled time, where this may be that themagnet field is monitored continually during operation of the MRIapparatus. When it is detected that the magnet field has decayed to alevel approaching a lower limit, an alert may be issued to an operatorwho may then schedule a ramp procedure at a next convenient opportunity.

An advantage of one or more of the present embodiments is that thecenter frequency of the body coil resonance curve is empiricallydetermined and used as a basis for computing the magnet targetfrequency. This is advantageous in, for example, low-field systems forwhich the body coil bandwidth is relatively narrow. A further advantageis that very little additional effort is required to implement themethod of one or more of the present embodiments. Instead of providing away to tune the body coil and another way to tune the magnet, one ormore of the present embodiments take the approach of using the body coilas it is, and tuning the magnet based on the body coil. Any drift of thebody coil center frequency (e.g., as a result of aging, from varyingmechanical load on the patient table of the body coil, etc.) will betaken into consideration in the target frequency for the magnet. Inother words, the magnet will always be ramped to suit the momentarystate of the body coil. The body coil will be tuned to within aspecified frequency bandwidth.

According to one or more of the present embodiments, the MRI apparatusincludes a body coil excitation unit for exciting a body coil to emit aradio-frequency signal. The MRI apparatus also includes a computationmodule (e.g., a processor) configured to determine the center frequencyof the radio-frequency signal emitted by the body coil, a targetfrequency determination unit (e.g., the processor or another processor)configured to calculate a magnet target frequency based on thedetermined center frequency, and a ramp control unit (e.g., theprocessor, the other processor, or yet another processor) configured toinitiate a ramping procedure to ramp the magnet to the target frequency.

Another advantage of the MRI apparatus of one or more of the presentembodiments is that the MRI apparatus may be significantly moreeconomical to run than a comparable prior art MRI apparatus, since atime interval between consecutive ramping events may be extended,resulting in less downtime (e.g., during which MRI scans cannot beperformed).

According to one or more of the present embodiments, the method ofoperating an MRI apparatus includes identifying a first magnet targetfrequency based on a first tolerance window and identifying a secondmagnet target frequency based on a second tolerance window. The secondtolerance window exceeds the first tolerance window. The method alsoincludes selecting one of the magnet target frequencies, and, at ascheduled time, ramping the magnet of the MRI apparatus to the selectedmagnet target frequency.

A target frequency for a ramping procedure is constrained by the upperboundary of the tolerance window defined by the bandwidth of theradio-frequency system as explained above. In the context of one or moreof the present embodiments, the first tolerance window has a differentbandwidth than the second tolerance window (e.g.,. one tolerance windowis narrower than the other). The two possible target frequencies willtherefore also be different. Depending on the nature of the scheduledramping procedure, the more appropriate of the two target frequencies isselected and used during the ramping procedure.

It may be assumed that the tolerance windows differ significantly (e.g.,that the width of the first tolerance windows differs from the width ofthe second tolerance window by a significant amount; by at least 10kHz-100 kHz).

According to one or more of the present embodiments, the MRI apparatusincludes a first frequency determination device (e.g., a processor) foridentifying a first magnet target frequency based on a first tolerancewindow, and a second frequency determination device (e.g., the processoror another processor) for identifying a second magnet target frequencybased on a second tolerance window, whereby the second tolerance windowexceeds the first tolerance window. The apparatus further includes aselection unit (e.g., the processor or another processor) configured toselect one of the magnet target frequencies as appropriate. A rampcontroller initiates a ramping procedure to ramp the magnet to theselected magnet target frequency.

An advantage of the MRI apparatus of one or more of the presentembodiments is that the MRI apparatus may be significantly moreeconomical to run, since the time interval between consecutive rampingevents may be extended, resulting in less downtime (e.g., during whichMRI scans cannot be performed).

As another example, a way of monitoring decay of the main magnetic fieldof an MRI apparatus is provided.

According to one or more of the present embodiments, the method ofoperating an MRI apparatus includes identifying a center frequencyduring successive imaging procedures (e.g., during patient scans) andrecording each center frequency, and analyzing the center frequencies toidentify a number of decay-related characteristics of the main magneticfield (e.g., the B0 field).

An advantage of one or more of the present embodiments is that the mainmagnetic field may be monitored during normal operation of the MRIapparatus without the need for any dedicated measuring devices oradditional components. One or more of the present embodiments are basedon the insight that the signal frequency detected at the receiver coilis related to the main magnetic field by the gyromagnetic ratio, andthat the signal detected by the receiver coil may be used to deduceinformation relating to the temporal development of the magnetic fieldduring successive scans. The field monitoring of the method isadvantageous in, for example, low-field systems, in which the magnetfield decay is to be remedied more frequently than an high-fieldsystems. Very little additional effort is required to implement themethod of one or more of the present embodiments, and essentially, allacts may be carried out by suitable algorithms that process signals thatare observed in any case during normal operation of the MRI apparatus.

An advantage of one or more of the present embodiments is that adecay-related characteristic of the main magnetic field is derivedentirely from measurements collected during patient scans (e.g., usingmeasurements that are made in any case). No dedicated test measurementsare to be made. The measurements may be collected while adjusting thefrequency of the receive system at the beginning of a patient scan, forexample. Alternatively or in addition, measurements may be collected ata point during the actual imaging sequence. In either case, the signalfor which the center frequency is determined originates from thepatient.

According to one or more of the present embodiments, the MRI apparatusincludes a center frequency identifier module configured to identify acenter frequency during an imaging procedure. The MRI apparatus alsoincludes a storage module configured to record center frequenciesidentified during successive imaging procedures, and an analysis moduleconfigured to identify a number of decay-related characteristics of themain magnetic field from analysis of the recorded center frequencies.

At least some of the units or modules of the MRI apparatus mentionedabove and below (e.g., the frequency determination unit and theselection unit) may be completely or partially configured as softwaremodules running on a processor of a control unit of an MRI apparatus. Aconfiguration largely in the form of software modules may have theadvantage that applications already installed on an existing MRI systemmay be updated, with relatively little effort, to carry out the methodacts of one or more of the present embodiments.

As yet another example, a computer program product with a computerprogram that is directly loadable into the memory of a control unit ofan MRI apparatus and includes program units to perform the acts of themethod of one or more of the present embodiments when the program isexecuted by the control unit is provided. In addition to the computerprogram, such a computer program product may also include further partssuch as documentation and/or additional components, also hardwarecomponents such as a hardware key (e.g., dongle, etc.) to facilitateaccess to the software.

A computer readable medium such as a memory stick, a hard-disk, or othertransportable or permanently-installed carrier may serve to transportand/or to store the executable parts of the computer program product sothat these may be read from a processor unit of an MRI apparatus. Aprocessor unit may include one or more microprocessors or equivalents.

The method of one or more of the present embodiments may be applied inany appropriate MRI apparatus (e.g., in an MRI apparatus that has asuperconducting magnet, a permanent magnet, or an electromagnet).However, a low-field MRI apparatus with a superconducting magnetbenefits to a greater extent from the method of one or more of thepresent embodiments. In the following, therefore, but withoutrestricting the present embodiments in any way, it may be assumed thatthe magnet of the MRI apparatus is a superconductive magnet.

It may also be assumed that the MRI apparatus is a low-field MRIapparatus. In one embodiment, the field strength of the magnet is atmost 1.0 T, at most 0.7 T, at most 0.5 T, or at most 0.3. As indicatedabove, the RF bandwidth of a mid- or high-field MRI apparatus isgenerally large (e.g., body coil bandwidth ±100 kHz or more), but thebandwidth of a low-field apparatus is significantly narrower (e.g., assmall as 10 kHz). In a further embodiment, the MRI apparatus has a bodycoil with a RF bandwidth that does not exceed 50 kHz.

An advantage of one or more of the present embodiments is that thetarget frequency of the magnet may be determined independently of animaging sequence or MR-experiment (e.g., which would require the mainmagnetic field B0) and is determined solely based on reflectionparameters of the body coil. In an embodiment, the magnet targetfrequency is determined in the absence of a main magnetic field. Inother words, the magnet target frequency may be determined even when themain magnet has been ramped down. One or more of the present embodimentsovercome a problem associated with low-field MRI systems (e.g., in suchlow-field systems, it is no longer practicable to specify the magnetfrequency independently of the body coil frequency). In contrast to ahigh-field systems with a body coil bandwidth large enough to include aseparately specified magnet frequency, the bandwidth of a low-fieldsystem is so narrow that the prior art approach of independentlyspecifying the magnet frequency runs the risk of not lying within thebody coil bandwidth.

To determine a suitable target frequency for the magnet in the next rampevent, the body coil center frequency is determined by measuring thereflection coefficients of the body coil as a function of frequency. Abody coil excitation unit excites the body coil to emit an RF signal ata set or chosen frequency. According to one or more of the presentembodiments, the center frequency of the body coil is deduced from asignal reflected by the body coil. The resonance curve of the reflectedsignal is detected and averaged in a suitable signal processing moduleto determine a corresponding center frequency. These computations may beperformed in the frequency domain.

The resonance curve may be established by using a directional coupler tomeasure the reflectivity of the body coil. Such a directional couplermay already be a component of the MRI system. However, some simple MRIsystems may not include such a directional coupler, or a directionalcoupler may be present but not equipped with the necessary detectors.Therefore, in a further embodiment, the center frequency of thereflected signal is determined by measuring the transmissivity betweenthe body coil and a further coil of the MRI apparatus. This may beachieved in a relatively straightforward manner by using a wide-bandtransmit antenna to transmit a signal and by detecting the receivedsignal. In one approach, this further coil may be a local coil of theMRI apparatus. Alternatively, the further coil may be a pickup coil ofthe MRI apparatus.

Once the body coil center frequency has been determined, the magnettarget frequency may be computed. In an embodiment, the magnet targetfrequency is adjusted by adding an offset to the determined body coilcenter frequency.

As explained above, the first tolerance window has a different bandwidththan the second tolerance window. In a further embodiment, the firsttolerance window is comparatively narrow and is applied when it ispossible to precisely determine the magnet frequency. In one embodiment,the first tolerance window has a bandwidth of, for example, at most 5kHz or at most 0.1 kHz. The first tolerance window may be applied whenthe magnet frequency may be estimated using a field probe, for example.By placing a field probe at a suitable position in the MRI apparatus(e.g., in the homogeneity volume or even into the isocenter of the B0field), it is possible to establish a direct relationship between thecurrent and/or voltage at the power supply and the detected echofrequency, thereby allowing the magnet frequency to be determined to ahigh degree of accuracy. The magnet may therefore be ramped to a targetfrequency near the upper boundary of the narrow tolerance window.Ramping to a higher target frequency will provide a longer duration(e.g., a wider decay window) before the magnet frequency decays to alower limit.

In a further embodiment, the second tolerance window is comparativelywide and has a bandwidth of, for example, at most 50 kHz or at most 10kHz. The second tolerance window therefore results in a lower targetfrequency. While this may appear to be a disadvantage, the advantage ofbeing able to estimate the magnet frequency using a low accuracy shuntand performing a significantly shorter ramp procedure may outweigh thedisadvantage of the shorter decay window.

The first tolerance window is used when the magnet is to be ramped to ahigh degree of precision using a field probe, for example.Alternatively, the first tolerance window is used when the magnet is tobe ramped using an array shim device (e.g., a specific arrangement offield probes that is used during an initial installation procedure toestablish a shim plot). This type of ramping procedure is more accuratebut it takes a long time for the magnet to reach the higher first targetfrequency. Such ramping procedures generally require expertise and areperformed by qualified personnel.

In one embodiment, the second target frequency is selected when themagnet is to be ramped in the absence of an MR echo frequency (e.g.,without using a field probe or measuring a reflected signal from thebody coil) or when the ramp procedure is to be performed without anyuser interaction. For example, a ramp procedure towards the secondfrequency may be performed using only the magnet power supply (MPSU)(e.g., by using an economical but inaccurate shunt to measure thecurrent or voltage of the MPSU while ramping and estimating themomentary value of the magnet frequency to determine when the rampsequence is complete). This less accurate ramping procedure takes ashorter time to reach the lower second target frequency and may bepreferred when a user is not available to give user input, as may occurwhen the magnet is to be ramped autonomously. An example of such ascenario may be when the magnet is to be ramped up after a power orcooling outage that resulted in the magnet being ramped down, sincepower is required for the cooling system. When power returns and thecooling system is once again operating reliably, the magnet may beautonomously ramped up without requiring any user to be present.

The tolerance windows to be used for the different target frequenciesmay be predefined (e.g., defined by the manufacturer). The variousparameters that define suitable tolerance windows may be supplied to thecustomer and may be stored in the system, or may be delivered with thesystem hardware, for example.

A target frequency is defined by the upper boundary of the body coil RFbandwidth and the appropriate tolerance window. This provides that thetarget frequency is always the highest possible for the next scheduledramping event. This helps to increase the average interval between rampevents over the lifetime of the MRI system and to reduce the systemdowntime.

The upper boundary of the RF bandwidth may be either set as a fixedparameter or may be measured during tune-up or during a manufacturingstage. The upper boundary of the RF bandwidth may be stored in software,for example, as a system parameter. Alternatively, the upper boundary ofthe RF bandwidth may be an individual parameter that is specified with amajor hardware component (e.g., the body coil) and is then read out fromthere by the central control system.

An advantage of one or more of the present embodiments is that either afirst ramping mechanism or a second ramping mechanism may be selectedbased on user input, or the system may establish which of the tworamping mechanisms is most appropriate, depending on which hardware isconnected to the system. For example, the presence of a shim array probemay be automatically recognized from a coil code that may be read by theMRI controller, which will then automatically select the narrowertolerance window. Ramp-up will be performed using field probes or theshim array device, and the reading of the MPSU sensor or shunt may becalibrated from the frequency measurement.

In one embodiment, during the ramp procedure, the magnet frequency isdetermined based on the magnet current as measured by a current sensor.The current sensor may include an ammeter shunt connected between theMPSU and the magnet. An ammeter shunt may measure large current valuesby a known and very small resistor in parallel with a moving coilgalvanometer. Such an ammeter shunt is economical, with a significantinaccuracy when new (e.g., on the order of 500 ppm). The inaccuracy mayincrease even further to 5,000 ppm as a result of long-term agingeffects. The magnet frequency may be derived from the current measuredby the shunt. Owing to the relatively poor accuracy of the shunt, themeasured magnet current may deviate from the magnet actual current by500-5,000 ppm, so that the derived magnet frequency will deviate fromthe actual magnet frequency by 500-5,000 ppm. Even so, this roughestimation of the magnet frequency may allow a target frequency to bedetermined for a ramp procedure. Using such a rough estimate for themagnet current is a trade-off against the longer time it would take toramp to a higher target frequency.

As an alternative to using a low-accuracy current sensor to estimate themagnet current and to derive the magnet frequency, the magnet frequencymay be estimated based on a field measurement from a Hall sensor. Herealso, the magnet frequency is derived in a comparatively inaccuratemanner, since a Hall sensor may generally only measure the B0 field towithin a limited accuracy, and the estimated field strength may differby 500-50,000 ppm from the actual field strength.

In an embodiment, a center frequency is determined during an imagingprocedure from signals received by the body coil or the local coils.These signals are essentially the signals originating from the patientduring a clinical imaging sequence.

As indicated above, the main magnetic field will inevitably decay overtime. While a system may be manufactured to a high degree of accuracy,and even though the magnet characteristics may be very preciselyquantified, it is not feasible to predict exactly how a magnetic fieldwill behave over several months or even years. However, it would bedesirable for the customer (e.g., the radiology department of a clinic)to be informed of the status of the magnet at any time, so that routinemaintenance events may be scheduled in a timely manner and so thatdown-time may be avoided.

In a method of one or more of the present embodiments, the recordedcenter frequencies may be stored in a memory module and/or may beforwarded to a remote service database for remote monitoring of thesystem. An alert or notification may be issued to the customer or userwhen the frequency approaches or transcends the limits of a predefinedtolerance window.

The recorded center frequencies together with the maximum bandwidth andthe decay-related characteristics make it possible to determine whetheror not a center frequency is still within a predefined window, and/or topredict when the center frequency will leave the predefined window. Awarning may be issued to the user or the customer service in good time(e.g., a few months or weeks ahead of the predicted time).

Therefore, in an embodiment, a decay-related characteristic includes arate of decay of the main magnetic field. The customer or an onlinesupervision system of a service IT infrastructure may compare thisinformation with the magnet specification to see whether the rate ofdecay is in spec (e.g., the magnetic field is decaying as predicted bythe manufacturer) or whether the rate of decay deviates from thepredicted rate. Particularly in the case of a higher decay rate, it isadvantageous to be aware of this so that corrective measures may beplanned accordingly. In a further embodiment, a decay-relatedcharacteristic may be a decay level of the main magnetic field (e.g.,the momentary percentage of the nominal field strength). The decay rateis effectively the rate of decay of the magnetic field (e.g., a firstderivative (dB/dt)). Another decay characteristic that may be ofinterest is the rate of change of the decay rate (e.g., the secondderivative (d²B/dt²)).

As explained above, a center frequency is obtained when a patient scanis being carried out. Occasionally, it may happen that patient scan isnot performed entirely correctly (e.g., when a metallic object is withinthe homogeneity volume, when the door of the radio-frequency cabin isinadvertently left open, etc.), so that the measured center frequency isnot actually representative. Therefore, in an embodiment, the methodincludes an act of filtering an identified center frequency prior tostoring the identified center frequency. For example, a condition may bethat an identified center frequency may not differ by more than ±50 ppmfrom a preceding center frequency. The filter is effectively aplausibility check that provides that outliers are not included in therecord of center frequencies.

After subjecting an identified center frequency to the plausibilitycheck, the identified center frequency is recorded in a memory. Arepresentative center frequency value may be obtained by averaging thecenter frequencies (e.g., filtered center frequencies) measured duringall patient scans during a specified time (e.g., during one day). Theanalysis unit may process a set of representative center frequencies todetect any decay of the main magnetic field. In one embodiment, theanalysis unit processes, for example, at least two representative centerfrequencies or at least ten representative center frequencies to derivea decay characteristic of the main magnetic field. For example, onerepresentative center frequency value may be obtained per day byaveraging the center frequencies measured during all patient scans madethat day, and the decay-related characteristic may be computed at theend of the week. Similarly, one representative center frequency valuemay be obtained from all patient scans made during one week, and thedecay-related characteristic may be computed at the end of the month.

If a decay-related characteristic exceeds a predefined threshold (e.g.,the magnet center frequency has dropped below a certain level) or thedecay rate has increased beyond a certain acceptable rate, the operatorof the MRI apparatus may be informed. Therefore, in an embodiment, themethod includes issuing an alert when a decay-related characteristicexceeds a predefined threshold.

The method of one or more of the present embodiments may be used toadvantage to observe the magnet field strength over time, and to planany corrective measures such as a partial ramp or automatic ramp. Forexample, analysis of the center frequencies may indicate that the centerfrequency of the magnetic field has decayed to a level that may beremedied by carrying out a partial ramp-up procedure that may be carriedout in the evening (e.g., when the MRI apparatus is not in use). Apartial or “top-up” ramp may take only a relatively short period of timecompared to a complete ramp-up procedure that requires the magnet to beramped down completely and then ramped up again to reach the desiredmagnet field strength.

During ramp-up, current is fed to the magnet until the magnet current orthe corresponding frequency has reached a predefined target current orfrequency. The target frequency may be determined in any suitable manner(e.g., by arranging a probe inside the body coil and monitoring themagnet current using a current sensor of the magnet power supply). TheMRI apparatus may include a probe placement device for automaticallyplacing the probe inside the body coil.

In an embodiment, the MRI apparatus includes an ammeter shunt arrangedto measure magnet current while ramping, so that the magnet frequencymay be estimated. When the magnet frequency has been observed to reachthe pre-defined target frequency, it may be assumed that the magneticfield is satisfactory and the ramp procedure may be concluded. Anammeter shunt is an economical but low-accuracy component and tends tobecome less accurate as the ammeter shunt ages. Therefore, in a furtherembodiment, the MRI apparatus includes a shunt monitoring module (e.g.,the processor or another processor) configured to derive an agingcharacteristic of the current sensor based on patient scan centerfrequencies. A current measurement may be recorded at the end of aramp-up sequence (e.g., when the magnet frequency has reached a targetfrequency). A calibration factor for the current sensor may be derivedfrom a relationship between the shunt current measurement and asubsequent center frequency. The calibration factor may be expressed asa simple ratio (e.g., shunt current divided by center frequency).

In one embodiment, a shunt current measurement and a patient scan centerfrequency measurement are temporally close. Since the shunt currentmeasurement may only be made during a ramp-up procedure, the centerfrequency measurement may be made during the patient scan following theramp-up.

The calibration factor may be used to correct the current sensorreadings during a subsequent ramp-up sequence, for example, and maysignificantly improve the accuracy of the ramp-up sequence, therebyextending the magnet field decay window. The calibration factor may alsobe used to correct for long-term aging effects of the shunt. Thecalibration factor may be used by the algorithms that deduce the magnetfrequency from the current sensor measurements. In this way, the MRIapparatus may re-calibrate this functionality in a straightforwardmanner and prior to a next scheduled ramp-up procedure.

In a further embodiment, any acts of analyzing and evaluating therecorded center frequencies are performed remotely (e.g., on an externalserver). A remote customer support service may analyze the collectedinformation and schedule a ramp-up event for MRI apparatus, so that thecustomer does not need to be concerned about these tasks.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparentfrom the following detailed descriptions considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for the purposes of illustration and not asa definition of the limits of the invention.

FIG. 1 shows a simplified circuit diagram of a superconductive low-fieldmagnetic resonance imaging (MRI) apparatus according to an embodiment;

FIG. 2 shows a simplified block diagram of an MRI apparatus according toan embodiment;

FIG. 3 illustrates a determination of a magnet target frequency using anembodiment of a method;

FIG. 4 shows a simplified circuit diagram of a superconductive low-fieldMRI apparatus according to an embodiment;

FIG. 5 shows a simplified block diagram of an MRI apparatus according toan embodiment;

FIG. 6 illustrates an exemplary determination of a target frequencybased on a narrow tolerance window;

FIG. 7 illustrates an exemplary determination of a target frequencybased on a wide tolerance window;

FIG. 8 shows a simplified block diagram of an MRI apparatus according toan embodiment;

FIG. 9 shows a simplified circuit diagram of a superconductive low-fieldMRI apparatus according to an embodiment;

FIG. 10 shows an exemplary graph of magnet field decay obtained using anembodiment; and

FIG. 11 shows a first graph of exemplary patient scan center frequenciesobtained using an embodiment of a method.

DETAILED DESCRIPTION

In the diagrams, like numbers refer to like objects throughout. Objectsin the diagrams are not necessarily drawn to scale.

FIG. 1 shows a greatly simplified circuit diagram of one embodiment of asuperconductive low-field MRI apparatus 1 (e.g., an MRI apparatus or anMRI system). The MRI apparatus 1 includes various modules and units,most of which will be known to the skilled person and need not beexplained here. The MRI apparatus 1 includes a main magnet 10 thatgenerates a very homogenous main magnetic field B0. An MPSU 10P is usedto supply current I₁₀ to the magnet 10 during a ramp-up procedure whenthe magnet 10 is ramped to a previously determined target frequency. Aswitch assembly 17 including a superconducting switch in parallel with abypass resistor is shown connected across the main magnet coil. Theswitch is closed during the ramp-up procedure so that a small amount ofcurrent passes through the bypass resistor. In this exemplaryembodiment, an ammeter shunt S is used to measure the magnet current I₁₀during ramping so that a power supply controller 15 may estimate themagnet frequency and compare the estimated magnet frequency to thetarget frequency f_(T) so that the ramp-up procedure may be halted whenthe target frequency f_(T) has been attained. At this point, the switch17 is opened again.

FIG. 2 shows a simplified block diagram of one embodiment of the MRIapparatus 1 indicating the main magnet 10 and the body coil BC. Theusual arrangement of additional coils such as shim coils, local coil,pickup coil, and a number of gradient coils may be assumed to bepresent. The diagram shows a body coil excitation unit 11 configured toexcite the body coil BC to emit an RF signal at a chosen frequencyf_(BC). A reflected RF signal f_(BC)′ is detected, and a resonance curveof the reflected RF signal f_(BC)′ is analyzed in frequencydetermination module 14 (e.g., a processor) to identify a correspondingcenter frequency f_(c). The center frequency f_(c) is stored in a memorymodule 13 (e.g., a memory device) that may be realized as a memorymodule of the body coil BC, or as a memory module of a control unit(e.g., a controller, the processor, or another processor) of the MRIapparatus 1. The reflection coefficients of the system provide that thecenter frequency f_(c) of the reflected body coil signal f_(BC)′ islower than the body coil frequency f_(BC).

A target frequency computation module 12 (e.g., the processor or anotherprocessor) determines a magnet target frequency f_(T) based on theidentified center frequency f_(c). Depending on the type of ramp-upsequence that is to be carried out, an offset df may be added to thefrequency f_(c). In an exemplary process flow, the center frequencyf_(c) may be identified, for example, by the manufacturer or at somepoint during the lifetime of the MRI apparatus 1. Either way, the centerfrequency f_(c) is stored in the memory module 13. Before carrying out aramp-up sequence, the center frequency f_(c) is retrieved from thememory module 13 and adjusted as necessary or as desired by a suitableoffset df to give the target frequency f_(T), and the magnet is rampedto the target frequency f_(T).

A ramp control module is provided to initiate a subsequent rampingprocedure at a suitable time. The magnet power supply unit 10Paccordingly supplies current I₁₀ to the magnet 10 during the ramp-upprocedure in order to ramp the main magnet 10 to that target frequencyf_(T).

The units and modules described above may be realized as part of acentral control system of the MRI apparatus 1.

FIG. 3 shows an exemplary resonance curve of a body coil. The Y-axisshows reflection coefficients between 0 and 1. The X-axis showsfrequency offset in kHz, with 0 corresponding to the minimum reflectioncoefficient. Such a curve is obtained by averaging the reflected bodycoil signal. The shape of the resonance curve 30 is determined largelyby the Q-factor of the body coil. For the comparatively low magnetfrequency of a low-field MRI system, the body coil has a high Q-factorduring an imaging sequence (e.g., with a patient inside the body coil)on account of the low ohmic losses arising from lower conductivity. In alow-field MRI system, therefore, the quality of an imaging procedure isdependent on how well the magnet frequency and body coil frequency arematched. The lowest point or minimum of the resonance curve,corresponding to the center frequency fc of the reflected body coilsignal, is identified and used to arrive at a magnet target frequencyfor a subsequent ramp-up sequence.

The target frequency may be set as the identified center frequency fcthat was identified in the resonance curve of the body coil reflection.However, an offset may instead be added to the target frequency. Themagnitude of the offset may be chosen based on the shape of theresonance curve and/or on various parameters of the ramp-up sequence.For example, by identifying a maximum reflection coefficient asindicated in FIG. 3 , a resulting offset df may be identified.Generally, it is desired to set the magnet target frequency to be higherthan the body coil frequency. Therefore, the target frequency f_(T) maybe expressed as:

f _(T) =f _(c) +df   (1)

Alternatively, a fraction of the offset may be used (e.g., 25% of theoffset). In this case, the target frequency f_(T) may be expressed as

$\begin{matrix}{f_{T} = {f_{c} + \frac{df}{4}}} & (2)\end{matrix}$

To give an example, the center frequency f_(c) of the reflected signalmay be determined to be 20.0 MHz. Adding a suitable offset such as 50kHz, the target frequency f_(T) for the next ramp event is determined tobe 20.05 MHz using the above equation. In this way, the target frequencyf_(T) may be identified based on a desired accuracy of the intendedramping procedure. The method of one or more of the present embodimentsof using an echo experiment to determine the magnet target frequency isassociated with a favorably high degree of accuracy (e.g., with an errorof less than 1.0 kHz). In another example, the center frequency f_(c) ofthe reflected signal may be determined to be 30.1 MHz. Adding a suitableoffset such as 10 kHz, the target frequency f_(T) for the next rampevent is determined to be 30.11 MHz using the above equation.

Excitation of the body coil BC, measurement of the reflected body coilsignal f_(BC)′, and computation of the center frequency f_(c) and thetarget frequency f_(T) may be performed entirely independently of themain magnet field B0, so that the method of one or more of the presentembodiments may be carried out when the magnet 10 is ramped down.

FIG. 4 shows a greatly simplified circuit diagram of a superconductivelow-field MRI apparatus 1. The MRI apparatus includes the main magnet 10that generates a very homogenous main magnetic field B0. FIG. 5 shows asimplified block diagram of one embodiment of an MRI apparatus 1indicating the main magnet 10. The usual arrangement of additional(e.g., a body coil BC, a shim coil, and a number of gradient coils) maybe assumed to be present. An MPSU 10P is used to supply current I₁₀ tothe magnet 10 during a ramp-up procedure. A switch assembly 17 includinga superconducting switch in parallel with a bypass resistor is shownconnected across the main magnet coil. The superconducting switch isclosed during the ramp-up procedure so that a small amount of currentpasses through the bypass resistor. When the desired magnetic fieldstrength has been reached, the switch is opened.

In this exemplary embodiment, a current sensor S is configured as anammeter shunt and includes a shunt 18 and a galvanometer 19 arranged tomeasure current through the shunt 18. The measured current I₁₀′corresponds essentially to the current I₁₀ through the magnet 10.Adjusting for the slight loss through the shunt, the magnet current I₁₀may be determined to an accuracy of 500-5,000 ppm. The current sensorcomponents will age over time, and this aging may be quantified and usedto adjust the measured current value as appropriate.

The MRI system 1 may be delivered with a set of parameters such as abody coil RF bandwidth W_(BC) and tolerance windows W1, W2 for use incomputation of a target frequency. Any such parameters may be stored ina suitable memory module. FIG. 5 shows two frequency determinationmodules 20, 21, each of which is realized to identify a magnet targetfrequency f_(T1), f_(T2) based on a tolerance window W1, W2, a body coilRF bandwidth W_(BC), and an appropriate input variable. To compute thehigher first target frequency f_(T1), the input variable to the firstfrequency determination module 20 is a measured RF signal f_(BC)′reflected from the body coil BC when a probe is placed inside the bodycoil BC during ramp-up. To compute the lower second target frequencyf_(T2), the input variable to the second frequency determination module21 is magnet current I₁₀′ measured by the ammeter shunt S.

A selection module 22 (e.g., the processor or another processor) isprovided to select one of the computed magnet target frequencies f_(T1),f_(T2) as appropriate for the next re-ramp procedure. A ramp controlmodule 23 (e.g., the processor or another processor) is provided toinitiate a ramping procedure to ramp the main magnet 10 to the selectedmagnet target frequency f_(T1), f_(T2). The units and modules may berealized as part of a central control system of the MRI apparatus 1.

FIG. 6 is a graph of amplitude against frequency, illustrating thecomputation of a target frequency f_(T1) for a ramping procedure, basedon a narrow tolerance window W1. In this case, the upper boundary of thebody coil radio-frequency bandwidth W_(BC) may be either set as a fixedparameter or may be measured during a tuning procedure or during amanufacturing stage. For example, the body coil RF bandwidth W_(BC) maybe in the order of 50 kHz. This information may be stored in software asa system parameter or may be a parameter that is delivered with the bodycoil (e.g., in a memory such as an EEPROM or a flash memory that is partof the body coil system). The body coil RF bandwidth W_(BC) may be readout from such a memory module by a controller of the MRI apparatus.

In this comparatively accurate method, a probe is placed in the bodycoil BC, and the magnet frequency f₁ is calculated.

The first target frequency f_(T1) may then be expressed as

f _(T1) =f ₁+½W _(BC)−½W1   (3)

To give an example, the frequency f₁ of the magnet may be estimatedusing a probe to be about 20 MHz. Using a known body coil RF bandwidthW_(BC) of 30 kHz and the known narrow window W1 of 1 kHz, the targetfrequency f_(T1) for the next ramp event is determined to be 20.014500MHz using equation (3).

FIG. 7 is a graph of amplitude against frequency, illustrating thecomputation of a second target frequency f_(T2) for a ramping procedure,based on a wide tolerance window W2. The upper boundary of theradio-frequency bandwidth W_(BC) is known, as explained in FIG. 6 above.The wide tolerance window W2 is also known. The magnet frequency f₂ isestimated using the comparatively inaccurate current I₁₀′ measured usingthe shunt S of FIG. 4 .

The second target frequency f_(T2) may then be expressed as

f _(T2) =f ₂+½W _(B)−½W2   (4)

To give an example, the frequency f₂ of the magnet may be estimatedusing the shunt to be about 20 MHz. Using a known body coil RF bandwidthW_(BC) of 30 kHz and the known wide window W2 of 10 kHz, the targetfrequency f_(T2) for the next ramp event is determined to be 20.01 MHzusing equation (4).

This roughly computed target frequency f_(T2) may be used when anautonomous ramp-up sequence is to be performed or when there is no useravailable to place a field probe, for example.

One embodiment of the MRI apparatus 1 includes a main magnet thatgenerates a very homogenous main magnetic field B0. FIG. 8 shows asimplified block diagram of one embodiment of the MRI apparatus 1indicating the main magnet 10, the body coil BC, and a receiver coil RC(e.g., a flat receive-only spine coil). The usual arrangement ofadditional coils such as shim coils, local coil, pickup coil and anumber of gradient coils may be assumed to be present. During a patientscan, the body coil BC is excited to emit an RF signal, and the receivercoil RC detects a reflected RF signal 100. The reflected RF signal 100is processed by a module 38 (e.g., the processor or another processor)that performs appropriate signal processing steps to identify the centerfrequency f_(c) of the received signal 100. To exclude any outliers, afilter is applied in block 110, which may compare the present centerfrequency f_(c) to the previous center frequency. The present centerfrequency f_(c) is only approved if, for example, the present centerfrequency f_(c) does not differ from a predecessor by more than ±50 ppm.If approved, the present center frequency f_(c) is then stored in amemory module 40 (e.g., a memory device). These steps are repeated forsuccessive patient scans, so that ultimately, a collection of centerfrequencies accumulates in the memory module 40.

The frequency of a received RC signal is related to the magnetic fieldstrength by the relationship

$\begin{matrix}{f = {\frac{\gamma}{2\pi}B0}} & (5)\end{matrix}$

where γ is the gyromagnetic ratio. An analysis module (e.g., theprocessor or another processor) processes a plurality of thesuccessively collected center frequencies f_(c1), . . . , f_(c1) toidentify a trend. For example, a gradual shift in frequency indicates agradual decay of the main magnetic field. The analysis module maycompare an identified decay trend with an expected decay trend knownfrom the magnet specification, stored, for example, in a memory module130 (e.g., a memory device). A decision module 41 (e.g., the processoror another processor) may determine whether action is to be taken basedon the information provided by the analysis unit 42 (e.g., the processoror another processor). For example, the decision module may issue analert X if a field strength decay rate is observed to be faster than anexpected or specified decay rate.

A ramp control module 43 (e.g., the processor or another processor) isprovided to initiate a ramping procedure at a suitable time in order toramp the main magnet 10 to a target frequency f_(T). FIG. 9 shows agreatly simplified circuit diagram of an embodiment of such asuperconductive low-field MRI apparatus 1. An MPSU 10P is used to supplycurrent I₁₀ to the magnet 10 during a ramp-up procedure, when the magnet10 is ramped to the previously determined target frequency. A switchassembly 17 including a superconducting switch in parallel with a bypassresistor is shown connected across the main magnet coil 10. The switchis closed during the ramp-up procedure so that a small amount of currentpasses through the bypass resistor. In this exemplary embodiment, anammeter shunt S is used to measure the magnet current I₁₀ during rampingso that a power supply controller 15 may estimate the magnet frequencyand compare the estimated magnet frequency to the target frequency f_(T)so that the ramp-up procedure may be halted when the target frequencyf_(T) has been attained. At this point, the switch 17 is opened again.

Returning to FIG. 9 , a current measurement may be recorded at the endof a ramp-up sequence (e.g., when the magnet frequency has reached atarget frequency). A calibration factor for the current sensor may bederived from a relationship between the current measurement and asubsequent center frequency. A calibration factor C may be expressed asa simple ratio of shunt current to center frequency, such as, forexample

$\begin{matrix}{C = \frac{I_{10}}{f_{c}}} & (6)\end{matrix}$

The shunt current measurement and a patient scan center frequencymeasurement may be temporally close. Since the shunt current measurementmay only be made during a ramp-up procedure, the center frequencymeasurement may be made during the patient scan following the ramp-up.

The calibration factor C may be used in a subsequent ramp-up procedureto correct for aging effects of the shunt S. The calibration factor Cmay be forwarded to the controller 15 of the MPSU 10P, for example, sothat subsequent current readings may be corrected, and the ramp-upprocedure may be performed to a higher degree of accuracy.

The units and modules described above may be realized as part of acentral control system of the MRI apparatus 1.

FIG. 10 is a graph of magnet field decay showing magnet field strength(in Tesla) against time (in a timescale of months or even years). Thediagram shows an expected decay profile 50 and a deduced decay profile50′ that is established by analyzing a series of patient scan centerfrequencies. In this exemplary case, the decay profile 50′ shows afaster rate of decay than the expected decay profile 50 and may resultin a decision to carry out a re-ramp event sooner than originallyscheduled.

FIG. 11 indicates how a collection of patient scan center frequenciesmay be used to establish a magnet field decay characteristic. Theidentified center frequencies f_(c1), . . . , f_(cn) are indicated bydots. The graph shows frequency on the X-axis (in MHz) against scancount along the Y-axis. The diagram shows a gradual decrease in thecenter frequencies f_(c1), . . . , f_(cn) of the detected RC signal.Using an appropriate interpolation or best-fit technique, thefrequencies may be translated into a decay profile 50′ of the mainmagnetic field as shown in FIG. 10 .

Although the present invention has been disclosed in the form ofpreferred embodiments and variations thereon, it will be understood thatnumerous additional modifications and variations could be made theretowithout departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of “a” or“an” throughout this application does not exclude a plurality, and“comprising” does not exclude other steps or elements. The mention of a“unit” or a “module” does not preclude the use of more than one unit ormodule.

1. A method of operating a magnetic resonance imaging (MRI) apparatus,the method comprising: identifying a center frequency during an imagingprocedure and recording center frequencies identified in successiveimaging procedures; and identifying a number of decay-relatedcharacteristics of a main magnetic field of the MRI apparatus, theidentifying of the number of decay-related characteristics of the mainmagnetic field comprising analyzing the center frequencies.
 2. Themethod of claim 1, wherein analyzing the center frequencies comprisesaveraging a plurality of center frequencies measured during patientscans during a specified interval.
 3. The method of claim 1, wherein adecay-related characteristic of the number of decay-relatedcharacteristics comprises a rate of decay of the main magnetic field. 4.The method of claim 3, wherein a decay-related characteristic of thenumber of decay-related characteristics comprises a rate of change ofthe rate of decay of the main magnetic field.
 5. The method of claim 1,further comprising filtering prior to storing the identified centerfrequency.
 6. The method of claim 1, further comprising issuing an alertwhen a decay-related characteristic of the number of decay-relatedcharacteristics exceeds a predefined threshold.
 7. The method of claim1, further comprising deriving an aging characteristic of a currentsensor from a current measurement obtained during a ramp-up sequence anda center frequency identified after that ramp-up sequence.
 8. The methodof claim 1, wherein analyzing the center frequencies is performed at aremote location from the MRI apparatus.
 9. A magnetic resonance imaging(MRI) apparatus comprising: a center frequency identifier moduleconfigured to identify a center frequency during an imaging procedure; astorage device configured to record center frequencies identified duringsuccessive imaging procedures; a processor configured to identify anumber of decay-related characteristics of a main magnetic field fromanalysis of the recorded center frequencies.
 10. The MRI apparatus ofclaim 9, further comprising: an ammeter shunt arranged to measure magnetcurrent during a ramp procedure; and a shunt monitoring moduleconfigured to derive an aging characteristic of the ammeter shunt basedon consecutive identified center frequencies.
 11. The MRI apparatus ofclaim 9, wherein a field strength of a magnet of the MRI apparatus is atmost 1.0 Tesla.
 12. The MRI apparatus of claim 11, wherein the magnet isa superconductive magnet.
 13. In a non-transitory computer-readablestorage medium that stores instructions executable by one or moreprocessors to operate a magnetic resonance imaging (MRI) apparatus, theinstructions comprising: identifying a center frequency during animaging procedure and recording center frequencies identified insuccessive imaging procedures; and identifying a number of decay-relatedcharacteristics of a main magnetic field of the MRI apparatus, theidentifying of the number of decay-related characteristics of the mainmagnetic field comprising analyzing the center frequencies.
 14. Thenon-transitory computer-readable storage medium of claim 13, whereinanalyzing the center frequencies comprises averaging a plurality ofcenter frequencies measured during patient scans during a specifiedinterval.
 15. The non-transitory computer-readable storage medium ofclaim 13, wherein a decay-related characteristic of the number ofdecay-related characteristics comprises a rate of decay of the mainmagnetic field.
 16. The non-transitory computer-readable storage mediumof claim 15, wherein a decay-related characteristic of the number ofdecay-related characteristics comprises a rate of change of the rate ofdecay of the main magnetic field.
 17. The non-transitorycomputer-readable storage medium of claim 13, wherein the instructionsfurther comprise filtering prior to storing the identified centerfrequency.
 18. The non-transitory computer-readable storage medium ofclaim 13, wherein the instructions further comprise issuing an alertwhen a decay-related characteristic of the number of decay-relatedcharacteristics exceeds a predefined threshold.
 19. The non-transitorycomputer-readable storage medium of claim 13, wherein the instructionfurther comprise deriving an aging characteristic of a current sensorfrom a current measurement obtained during a ramp-up sequence and acenter frequency identified after that ramp-up sequence.
 20. Thenon-transitory computer-readable storage medium of claim 13, whereinanalyzing the center frequencies is performed at a remote location fromthe MRI apparatus.